1. Field of the Invention
This invention relates generally to methods of forming nanoscale structures and the nanoscale structures formed thereby. In particular, this invention relates to methods using multi-photon excitation for the fabrication of structures with nanometer-level precision.
2. Description of the Related Art
Three-dimensional objects having fine-scale microstructures possess unique and technologically attractive properties. There has been particular interest recently in the fabrication of structures with nanometer-level precision, that is, objects with structures or structural features measurable in the nanometer range. Such nanoscale structures have dimensions or features in the range of about 2 to about 100 nm (nanometer, wherein 1 nm=10 angstroms), which is on the order of the size of macromolecules such as proteins and protein complexes.
Photolithography, including methods using X-ray and deep UV, is well-known for producing two-dimensional structures with small-scale features. However, this technique does not allow the production of complex, curved three-dimensional surfaces, as it is very limited in the complexity achievable in the z-direction. Three-dimensional objects produced by photolithographic methods have therefore been essentially limited to columnar structures. Objects with features smaller than 150 nm are not readily producible or routinely available. George M. Whitesides has also described several methods for micro-scale fabrication based on microcontact printing and modification of surface chemistry with self-assembled monolayers. These methods, however, are also very limited in the ability to build in the third dimension, as well as in their chemistry. A method for manufacturing three-dimensional optical data storage and retrieval structures by reaction of polyesters using two-photon excitation is disclosed in U.S. Pat. No. 5,289,407 to Strickler, et al., which is incorporated herein by reference in its entirety.
A number of other, different approaches have been described for the synthesis of three-dimensional objects with small-scale features, for example biomimetic matrix topographies such as basement membrane textures. As described in U.S. Pat. No. 5,700,241, such structures are produced by removal of epithelial or endothelial cell layers to expose the supporting basement membrane or matrix. The exposed topography is then used as a mold for polymer casting. The surface of the resultant molded negative replica of the matrix topography is then itself cast with the final (bio)material of choice. With this methodology, three-dimensional bionminetic matrices can be prepared for both experimental investigations into cellular biology, and to potentially improve the cell and tissue response to implanted biomaterials. Although this method can produce very complex three-dimensional topography, it does not provide for topographic design flexibility, since all constructs must begin with a biological surface. In addition, while many materials may be used for fabrication, the procedure does not provide for spatial control of chemistry.
Scanning tunneling microscopy has also been used to move atoms on surfaces. However, this technique is extremely limited in the sizes and chemistry of the fabricated region. Another technique which has been described for the solid, free-form fabrication of microscale structures includes forming successive, adjacent, cross-sectional laminae of the object at the surface of a fluid medium or other bed, the successive laminae being automatically integrated as they are formed to define the desired three-dimensional object, as disclosed in U.S. Pat. No. 4,575,330 to Hull. U.S. Pat. No. 5,518,680 to Cima et al. similarly discloses successive printing of layers of powder in a solvent which causes binding of the successive layers, thereby allowing the formation of drug delivery devices having thicknesses on the order of about 100 microns.
Three-dimensional objects have also been generated by selective curing of a reactive fluid medium by a beam or beams of ultraviolet (UV) radiation brought to selective focus at prescribed intersection points within the three-dimensional volume of the fluid medium. Disadvantages of such systems include the use of UV radiation, which requires expensive and cumbersome optics and lens, as well as the associated poor focusing qualities of excimer and other UV laser sources.
An additional technique for generating three-dimensional microscale objects is described by S. Maruo, O. Nakamura, and S. Kawata et al. in "Three Dimensional Microfabrication With Two-Photon-Absorbed Photopolymerization", Optics Letters, Vol. 22, No. 2, pp. 132-134 (1997), which is incorporated herein by reference in its entirety. Maruo et al. discloses that microscale structures are formed by subjecting urethane acrylate monomers and oligomers to near-irfrared laser light in a non-solvent system. Use of two-photon absorption for initiation of the reaction leads to a spiral wire having a diameter of 6 microns, an axial pitch of 10.3 micron, and a width of nearly 1.3 microns. While small, such structures are not in the nanoscale region. Maruo et al. furthermore only describe synthesis in a non-solvent system, which is incompatible with most biomolecules.
Accordingly, there stiff remains a need for methods of free-form fabrication of two- and three-dimensional structures having dimensions or features in the micron and nanometer range, especially techniques suitable for synthesis using biomolecular subunits such as proteins, peptides, oligonucleotides, as well as bio-active small molecules such as hormones, cytokines and drugs.